1. Field of the Invention
This invention relates to the field of inertial guidance and more particularly to the field of ring laser gyroscopes and to detector systems used by ring laser gyroscopes to detect rotational information from counterpropagating light beams.
2. Description of Prior Art
Detector systems used in ring laser gyroscope applications typically employ at least one partially transmissive mirror at locations in a resonant cavity through which components of a CW and a CCW beam are extracted. The beams are then combined using combining optics such as a prism. The combined beams are then directed onto a focal plane to form an illuminated spot in which interference patterns are characterized as areas of high and low intensity. As a ring laser gyroscope experiences an input body rate, the interference patterns move across the illuminated spot in response to the SAGNAC effect.
U.S. Pat. No. 4,514,832 titled "Single Mirror Ring Laser Gyro Readout Without Combining Optics", and U.S. Pat. No. 4,514,087 titled "Ring Laser Gyroscope Readout for Partially Overlapping Beams", each of these patents being issued on Apr. 30, 1985, and having a common inventor and assignee with this application, each characterize laser gyroscope detector systems different from this invention.
Conventional ring laser gyroscope detector systems position an array of two or more PIN diode detectors in the illuminated spot on the focal plane, the diode detectors being positioned and biased to sense and provide electrical signals in response to movement of the interference pattern across their surface. The electrical signals thus provided are amplified and conditioned to provide digital body rate information.
A conventional ring laser gyroscope readout detector system will typically have an illuminated spot size of greater than 0.020 inches and will use diodes of length greater than the spot size, width small compared to fringe spacing, and spacing in the 10 milli-inch range.
The diodes used are typically formed from semiconductor material such as doped silicon and typically are rectangular in shape. The size of the diode, the depth of the diffusion, the peak intensity of the illuminated spot and the separation of the interference patterns each combine to influence the detector system's signal to noise ratios and bandwidth.
FIG. 1 shows a PRIOR ART prism and mirror arrangement for extracting a sample of the two counter-rotating beams from the cavity of an RLG. The optical elements shown include partially transmissive MIRROR 118 on substrate 120. A small percentage of the CCW (counter-clockwise) beam 116 passes through MIRROR 118 and is characterized as ray 122. A small percentage of the CW (clockwise) beam 112 passes through MIRROR 118 and is characterized as ray 132.
As shown in FIG. 1, the CW beam 132 passes through prism 110 without deviation as ray 128. The CCW beam 122 is totally internally reflected twice inside the prism before it impinges on a beam splitting coating on the surface between F and G. Angles shown are intended to be only illustrative of one particular embodiment, other combinations of angles being possible.
At the point of interception of ray 126 with beam splitting coating at FG, its reflection ray 130, makes a small angle with the transmitted CW beam 128. A small angle e is formed between the combined beams after they pass through prism surface HJ, and a fringe pattern is produced. Section line 1a--1a is taken through a sight plane as viewed by eye 108.
FIG. 1a depicts the image 130 of sight plane 1a--1a. Stippled (dotted) stripe regions 134, 136 are intended to characterize dark regions while regions 138, 139 and 140 are intended to characterize illuminated regions.
The dark stripes 132, 134 move laterally in a + or -x direction depending on the sense of the rotation rate of the gyro body. The dark stripes retain their co-parallel relationship in line with the y axis at all times.
The fringe spacing d is given by: EQU d=.lambda./e where
.lambda. is the laser operating wave length and e is the angle between the two beams. With a zero input rate to the gyro, the fringe pattern of FIG. 1a is stationary. With a non-zero input rate, the fringe pattern moves at the gyro output beat frequency rate determined by its scale factor in either the + or -x directions dependent upon the sense of the input rotational rate.
Referring now to FIG. 2 shows the detector system of FIG. 1 with the addition of detector assembly 150 on surface HJ of PRISM 110.
Detector assembly 150 receives beams 128 and 130 through aperture 129 (not shown) as spot 130. Spot 130 is formed on the detector focal plane of section line 2a--2a.
FIG. 2a shows SPOT 130 on the focal plane. Detectors 142 and 144 are shown formed on the focal plane of 2a--2a.
Detectors 142, 144 are co-parallel with each other and with striped regions 136, 134. Movement of these striped regions in the +x or -x direction is sensed by the detectors 142, 144 and amplified by respective amplifiers (not shown). The detectors are formed to contain bias circuitry for the detectors and noise amplifiers for the sensed signal within the detector system of enclosure 150.
The sense of the input body rate is determined from the signals from each of the two detectors by spacing the detectors 90 degrees apart on the fringe pattern as shown in FIG. 2. The phase relationship between the signals detected at detectors 142 and 144 is a function of time and the sense of the input body rate.
The phase relationship thus serves to indicate the direction of the fringe motion. Electronic circuits such as those characterized in an application, filed 1/14/83, by A. K. Dorsman, Ser. No. 457,845, titled "Apparatus for Increasing the Resolution of A Laser Gyroscope", under U.S. Patent Office Secrecy Order, and having a common assignee, are used to process the detector signals from diodes 142 and 144 and to produce pulses indicating angular rate and MODE pulses indicating the sense of rotational rate. This application is presently under Secrecy Order dated May 16, 1985 and is not otherwise classified.
The PRIOR ART systems of FIG. 2 typically require that very tight tolerances be imposed on the manufacture of SUBSTRATE 120 and PRISM 110. Assembly tolerances are also very critical. The cement joint of space DKEG is typically limited in angular deflection to less than five (5) arc seconds. These tight tolerances are necessary to achieve fringe spacing in the 0.050 inch range to permit convenient detector size and spacing d.